Metal-air cells have been recognized as a desirable means for powering portable electronic equipment, such as personal computers, camcorders and telephones, because such battery cells have a relatively high power output with relatively low weight as compared to other types of electrochemical battery cells. Metal-air batteries include an air permeable cathode, commonly referred to as an oxygen electrode, and a metallic anode separated by an aqueous electrolyte. Electrical energy is created with a metal-air battery by an electrochemical reaction.
Metal-air battery cells utilize oxygen from the ambient air as a reactant in the electrochemical process. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the oxygen electrode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air cells utilize oxygen from the ambient air as a reactant, rather than utilizing a heavier material, such as a metal or metallic composition. To operate a metal-air battery, it is therefore necessary to provide a supply of oxygen to the oxygen electrode of the battery.
To preserve the efficiency, power and lifetime of a metal-air cell, it is desirable to effectively isolate the oxygen electrodes and anode of the metal-air cell from the ambient air while the cell is not operating. For example, U.S. Pat. No. 5,691,074 to Pedicini, entitled "DIFFUSION CONTROLLED AIR VENT FOR A METAL-AIR BATTERY", the entire disclosure of which is incorporated herein by reference, discloses systems for controlling the isolation of one or more metal-air cells from the ambient air while the cells are not operating. In accordance with one example of that which is disclosed by Pedicini, a group of metal-air cells are isolated from the ambient air, except for an inlet passageway and an outlet passageway. These passageways may be, for example, elongate tubes. An air moving device circulates air across the oxygen electrodes and forces air through the inlet and outlet passageways to refresh the circulating air with ambient air, so that oxygen is supplied to the oxygen electrodes. The passageways are sized to (i) pass sufficient airflow while the air moving device is operating to enable the metal-air cells to provided an output current for powering a load, but (ii) restrict airflow while the passageways are unsealed and no air is forced therethrough by the air moving device, so that a limited amount of air diffuses through the passageways. In this latter state, the oxygen electrodes are sufficiently isolated from the ambient air so that the cells have long "shelf life" without sealing the passageways. The passageways may be referred to as "isolating passageways" or "diffusion limiting passageways" due to their isolating capabilities.
In accordance with the above-referenced example from Pedicini, the isolating passageways function to limit the amount of oxygen that can reach the oxygen electrodes. This isolation minimizes the self discharge and leakage or drain current of the metal-air cells. Self discharge can be characterized as a chemical reaction within a metal-air cell that does not provide a usable electric current. Self discharge diminishes the capacity of the metal-air cell for providing a usable electric current. Self discharge occurs, for example, when the zinc anode of a metal-air cell is oxidized by the oxygen remaining within the cell when the cell is turned off, or by oxygen that seeps into the cell during periods of non-use. Leakage current, which is synonymous with drain current, can be characterized as the electric current that can be provided to a closed circuit by a metal-air cell while air is not provided to the cell by an air moving device. Isolating passageways as described above may limit the drain current to an amount smaller than the output current by at least a factor of 50.
The isolating passageways also minimize the detrimental impact of humidity on the metal-air cells, especially while the air moving device is not forcing airflow through the isolating passageways. A metal-air cell that is exposed to ambient air having a high humidity level may absorb too much water through its oxygen electrode and fail due to a condition referred to as "flooding." Alternatively, a metal-air cell that is exposed to ambient air having a low humidity level may release too much water vapor from its electrolyte through the oxygen electrode and fail due to a condition referred to as "drying out." The isolating passageways limit the transfer of moisture into or out of the metal air cells while the air moving device is off, so that the negative impacts of the ambient humidity level are minimized.
The efficiency of the isolating passageways in terms of the transfer of air and water into and out of a metal-air cell can be described in terms of an "isolation ratio." The "isolation ratio" is the ratio of the rate of the water loss or gain of the cell while its oxygen electrodes are fully exposed to the ambient air, as compared to the rate of water loss or gain by a cell while its oxygen electrodes are isolated from the ambient air, except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity of approximately fifty percent (50%), the ambient air having a relative humidity of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air could be more than 100 times greater than the water loss from a cell having an oxygen electrode that is isolated from the ambient air, except through one or more isolating passageways of the type described above. In this example, an isolation ratio of more than 100 to 1 is implied.
Metal-air cells have found limited commercial use in devices, such as hearing aids, which require a low level of power. In these cells, the air openings which admit air to the oxygen electrode are so small that the cells can operate for some time without flooding or drying out as a result of the typical difference between the outside relative humidity and the water vapor pressure within the cell. However, the power output of such cells is too low to operate devices such as camcorders, cellular phones, or laptop computers. Enlarging the air openings of a typical "button cell" would lead to premature failure as a result of flooding or drying out.
Ventilation systems designed to provide the dual functions of providing air to a metal-air cell for power output and isolating the cells during non-use are referred to as air managers. An important component of a successful air manager is an air mover, such as a fan or an air pump. In the past, air movers used in metal-air batteries have been bulky and expensive relative to the volume and cost of the metal-air cells. While a key advantage of metal-air cells is their high energy density resulting from the low weight of the oxygen electrode, this advantage is compromised by the space and weight required by an effective air mover. Space that could otherwise be used for battery chemistry to prolong the life of the battery must be used to accommodate an air mover. This loss of space can be critical to attempts to provide a practical metal-air cell in small enclosures such as the "AA" cylindrical size now used as a standard in many electronic devices. Also, the air mover uses up energy stored in the cells.
One factor increasing the required output characteristics of an air mover for a metal-air cell is the need to overcome the flow resistance of isolating passages of the type described above. To allow smaller capacity air movers, there is a need for an air manager that permits greater ambient air flow to support higher power output while the metal-air battery cells are in use without making the air mover larger or more expensive to acquire or operate. This new air manager should also restrict the ambient air flow to the extent necessary to protect the cells against excess humidity exchange when the metal-air battery cells are no longer is use.
In response to these realized inadequacies, the present invention seeks to provide a primary metal-air power source that can provide intermittent use at high power levels over a long lifetime for portable electronic products. This primary power source must combine one or more high energy metal-air cells with diffusion air manager technology.